// find this from kotlin's doco. The seed is the previous result
fun fibs(): Sequence<Int> {
    // fibonacci terms
    return generateSequence(Pair(1, 2), { Pair(it.second, it.first + it.second) }).map { it.first }
}

// Example:
// projecteuler.net problem 2:
// fibs().takeWhile{ it < 4_000_000 }.filter { it % 2 == 0 }.sum()
//